The intervertebral disc is composed of three distinctively different tissues: annulus fibrosis, nucleus pulposus, and vertebral endplate. Treatment of spinal disc disorders is often attempted by replacement of the disc nucleus (prosthetic nuclear implantation). This procedure is of importance because it can preserve the remaining disc tissues, that is, the annulus and the endplates, and therefore can preserve their functions. The primary objectives of prosthetic nucleus implantation are to re-gain the disc space and to relieve the compressive load on this disc component by sharing a significant portion of that load.
U.S. Pat. No. 5,800,549 to Bao describes implanting one or several (four implants were used in one example) dehydrated prosthetic hydrogel nuclei into the intradiscal space. Subsequent to implantation these implanted nuclei hydrate to a shape conforming to the natural nucleus. Bao also describes an insertion device for inserting an elastic prosthetic spinal nucleus into the intervertebral space. The insertion device includes a force-transmitting element for a rapid deployment of the prosthesis into the disc space in order to prevent permanent deformation of the elastic prosthetic nucleus. These implants taught by Bao are relatively large and have to be compressed in order to pass through the device for subsequent hydration. The embodiments of Bao have several disadvantages. First, the implant's size increases as it hydrates to its maximum volume. It is not feasible to precisely match the implant size to the desired intradiscal space (former nucleus), and therefore it cannot exert the optimum pressure on the vertebrae. In addition, during the hydration process, the core-containing jacket occupies a smaller space than the intradiscal space and is free to move around into a less favorable location while it is hydrating. Moreover, high pressure must be exerted on the implant in order to pass it through the insertion device. This process can release the implant at a high velocity into the intradiscal space and possibly damage the annulus.
U.S. Pat. No. 6,022,376 to Assell describes a capsule shaped prosthetic spinal disc nucleus for implantation into a human intradiscal space, made of a substantially inelastic constraining jacket surrounding an amorphous core. The jacket according to Assell is percutaneously implanted into a damaged disc space through a flap created in the annulus. The jacket can be implanted empty and then subsequently filled with the core through the use of a syringe or a catheter, which is directed to pass through the constraining jacket, or alternatively a core-containing jacket can be implanted. In both of these cases according to Assell, a final capsule volume is achieved following the hydration of the hydrogel core. Thus, like Bao, the teachings of Ansell have an inherent disadvantage of a small implant volume at the time of implantation that increases in volume over time. Further, the length of the flap according to Assell is about 12 millimeters and has a height of about 6 millimeters for use with a prosthetic body having a minor axis diameter of 7 millimeters. The relatively large size of the flap is an inherent and insurmountable drawback of the Assell device. Further, during the time that hydration takes place the core-containing jacket occupies a smaller space than in its final position and is free to move around into a less favorable position.
U.S. Pat. No. 6,306,177 to Felt describes the use of a curable polyurethane biomaterial composition adapted to be mixed at the time of use in order to provide a flowable composition and to initiate cure. Although the system according to Felt seemingly solves the problem of not completely filling up the available space, several drawbacks exist in Felt's system. These drawbacks include, among others, the unknown biostability of the thermoset polyurethane, the intricate instrumentation needed for the in-situ curing and the curing time which may be too rapid (less than a few minutes) for any last minute modifications.
U.S. Pat. No. 6,676,665 to Foley et al. describes instrumentation for treatment of the spine, including an elongate member having a deformable distal end portion at least partially formed of a flexible and preferably elastic material. The distal end portion has an initial configuration for placement adjacent a vertebral body and a deformed configuration defining at least one outwardly extending projection for displacement of at least a portion of the vertebral body. The elongate member preferably comprises a rod member, a sleeve member and an actuator mechanism for imparting relative linear displacement between the rod and sleeve members to effect outward deformation of the distal end portion of the sleeve member. In one embodiment, the instrumentation is used to compact cancellous bone to form a cavity within a vertebral body. In another embodiment, the instrumentation is used to reduce a compression fracture. In yet another embodiment, the instrumentation is used to distract a disc space between adjacent vertebral bodies.
Although many prosthetic disc devices are described in the literature, there is still a need for improvement in ease of manufacture and performance.